Re: Paging out when free memory is low but not exhausted (and available memory remains high)

[Michal Hocko <mhocko@kernel.org>]

On Wed 22-01-20 00:40:12, Chris Edwards wrote:
> Hi all,
> 
> I've observed a strange behaviour in recent kernels whereby the system will page out, continuously and seemingly needlessly, when free (cf. available) memory is low but not exhausted. Strangely, swap space utilisation does not increase over time, despite the continuous page-outs with little or no page-in activity.
> 
> I've seen this on 5.3 and 5.4 series kernels on Arch Linux on a Dell Optiplex 9020 (Intel Q87 Express, i5-4670 CPU, 12 GiB RAM, no NUMA). I've also replicated the behaviour on a colleague's machine (same configuration).
> 
> 
> To reproduce:
> 
> 
> 1. Get the system in a low-memory state, but not so low that it has to start paging. A convenient way is to use stress/stress-ng:
> 
> 	stress --vm-bytes $(awk '/MemAvailable/{printf "%d\n", $2 * 0.95;}' < /proc/meminfo)k --vm-keep -m 1
> 
> 	Confirm that the system isn't paging, and that memory pressure reported by /proc/pressure/memory remains low:
> 
> 	cat /proc/pressure/memory 
> 	some avg10=0.00 avg60=0.00 avg300=1.54 total=414331152
> 	full avg10=0.00 avg60=0.00 avg300=1.42 total=377927386
> 
> 
> 2. Perform an additional task that will fill up the filesystem cache and/or I/O buffers, such as dd.
> 
> 	dd if=/dev/sda of=/dev/null bs=1048576
> 
> 
> 3. Observe system behaviour (with `vmstat 1`, `xosview`, etc.) while the free memory decreases. Here's what I witness using vmstat:

Could you collect /proc/vmstat every second or so while you observe this
behavior? This should give us more information that vmstat(8) output.

Thanks!
-- 
Michal Hocko
SUSE Labs